Electrophotographic processing is commonly performed by the following procedures: a latent electrostatic image is formed by various electrical means on a photoreceptor made of selenium or some other suitable photoconductive materials; and toner particles are deposited on the latent image by a suitable method of development such as magnetic brush method so as to produce a visible image.
In the development step, carrier particles are used in order to impart an appropriate amount of positive or negative electrical charges to the toner. Various types of carriers have so far been developed and used commercially.
While carriers are required to possess various characteristics, particularly important ones include appropriateness of the plurality of charges generated by electrification, high impact and wear resistance, efficiency in development, and long developer life. In these respects, the prior art carriers still have problems to be solved and an product having satisfactory characteristics has not yat been achieved. For example, iron oxide powders and other electrically conductive carriers are capable of producing solid developed images of high quality but are not equally effective in reproducing fine lines of good quality. Furthermore, they require a special charge control agent to be incorporated in the toner in order to extend the life of the developer. Developers using coated carriers (i.e., with an insulation coating) have an extended life and a capability of reproducing fine lines of good quality but they are ineffective in reproduction of solid developed images of high quality. With a view to solving these problems, carriers consisting of small-diameter particles in which fine magnetic particles are dispersed in a binder resin have been proposed and commercialized, as described in U.S. Pat. No. 4,345,014 and Japanese Patent Application (OPI) No. 66134/79 (the term "OPI" as used herein means "unexamined published application"). However, this is not a complete solution to the problem of the shortness of developer life because: small-diameter carrier particles have a great tendency to adhere to the surface of the photoreceptor, their chargeability varies depending upon the moisture content of the environment in which they are used, and they are not highly amenable to effective surface treatment.